SEXUAI. DIMORPHISM 



169 



P H L 

 (Length of shell compared — — with that of posterior hinge line.) 



A H I 

 (Length of shell compared r~^ with that of anterior hinge line.) 



TH 



(Thickness of shell divided j^y j-, by dorso-ventral diameter.) 



about in the median line. Female shell a little fuller and more 

 rounded in the post-basal region." Specimens of Paraptera were 

 not abundant, but if in this species "rhomboidal" may generally 

 imply a greater height compared with the length, there is some 

 agreement on the part of my results. Inflation of the post-basal 

 region was not measured. 



Proptera: "Female shell with long rounded marsupial swelling 

 at extreme post-basal part, generally sub-truncate behind. Male 

 shell less full in post basal region, nearly rounded behind." My 



